Thysanoessa spinifera is a common and widespread species of krill (krill are small shrimp-like crustaceans occurring in often extremely dense concentrations in all the world's oceans). Adults reach 3 cm in length (Fiedler et al. 1998; Brinton and Townsend 2003). Thysanoessa spinifera ranges from southeast Alaska to northern Baja California (Brinton 1962, cited in Ainley et al. 1996). A northern cold-water coastal species, it frequently ranged far south off Baja California before 1960, but became limited to Central California in the 1980s (Brinton and Townsend 2003). Thysanoessa spinifera occurs mostly in waters less than 100 m deep (Brinton 1962, cited in Fiedler et al. 1998), although later life history stages may occur deeper in the water column, e.g. 150-200 m (Croll et al. 2005 and references therein).

In a study of the feeding habits of the world's largest stock of blue whales (Balaenoptera musculus), near the Channel Islands off southern California, in the summers of 1995 and 1996, the krill species Thysanoessa spinifera and Euphasia pacifica were found to be a major component of the diet of blue whales, with a preference shown for the larger, more coastal E. spinifera (Fiedler et al. 1998). These same two krill species were found to dominate the diets of blue whales foraging in Monterey Bay (central coast of California, USA) (Croll et al. 2005). A blue whale may consume up to two tons of krill each day (Rice 1978, cited in Fiedler et al. 1998).